Apparatus and method for generating an error concealment signal using an adaptive noise estimation

ABSTRACT

An apparatus for generating an error concealment signal, includes: an LPC representation generator for generating a replacement LPC representation; an LPC synthesizer for filtering a codebook information using the replacement LPC representation; and a noise estimator for estimating a noise estimate during a reception of good audio frames, wherein the noise estimate depends on the good audio frames representation generator is configured to use the noise estimate estimated by the noise estimator in generating the replacement LPC representation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending InternationalApplication No. PCT/EP2015/054486, filed Mar. 4, 2015, which isincorporated herein by reference in its entirety, and additionallyclaims priority from European Applications Nos. EP EP14160774.7, filedMar. 19, 2014, EP 14167003.4, filed May 5, 2014 and EP 14178761.4, filedJul. 28, 2014, which are all incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION

The present invention relates to audio coding and in particular to audiocoding based on LPC-like processing in the context of codebooks.

Perceptual audio coders often utilize linear predictive coding (LPC) inorder to model the human vocal tract and in order to reduce the amountof redundancy, which can be modeled by the LPC parameters. The LPCresidual, which is obtained by filtering the input signal with the LPCfilter, is further modeled and transmitted by representing it by one,two or more codebooks (examples are: adaptive codebook, glottal pulsecodebook, innovative codebook, transition codebook, hybrid codebooksconsisting of predictive and transform parts).

In case of a frame loss, a segment of speech/audio data (typically 10 msor 20 ms) is lost. To make this loss as less audible as possible,various concealment techniques are applied. These techniques usuallyconsist of extrapolation of the past, received data. This data may be:gains of codebooks, codebook vectors, parameters for modeling thecodebooks and LPC coefficients. In all concealment technology known fromstate-of-the-art, the set of LPC coefficients, which is used for thesignal synthesis, is either repeated (based on the last good set) or isextra-/interpolated.

ITU G.718 [1]: The LPC parameters (represented in the ISF domain) areextrapolated during concealment. The extrapolation consists of twosteps. First, a long term target ISF vector is calculated. This longterm target ISF vector is a weighted mean (with the fixed weightingfactorbeta) of

-   -   an ISF vector representing the average of the last three known        ISF vectors, and    -   an offline trained ISF vector, which represents a long-term        average spectral shape.

This long term target ISF vector is then interpolated with the lastcorrectly received ISF vector once per frame using a time-varying factoralpha to allow a cross-fade from the last received ISF vector to thelong term target ISF vector. The resulting ISF vector is subsequentlyconverted back to the LPC domain, in order to generate intermediatesteps (ISFs are transmitted every 20 ms, interpolation generates a setof LPCs every 5 ms). The LPCs are then used to synthesize the outputsignal by filtering the result of the sum of the adaptive and the fixedcodebook, which are amplified with the corresponding codebook gainsbefore addition. The fixed codebook contains noise during concealment.In case of consecutive frame loss, the adaptive codebook is fed backwithout adding the fixed codebook. Alternatively, the sum signal mightbe fed back, as done in AMR-WB [5].

In [2], a concealment scheme is described which utilizes two sets of LPCcoefficients. One set of LPC coefficients is derived based on the lastgood received frame, the other set of LPC parameters is derived based onthe first good received frame, but it is assumed that the signal evolvesin reverse direction (towards the past). Then prediction is performed intwo directions, one towards the future and one towards the past.Therefore, two representations of the missing frame are generated.Finally, both signals are weighted and averaged before being played out.

FIG. 8 shows an error concealment processing in accordance withconventional technology.

An adaptive codebook 800 provides an adaptive codebook information to anamplifier 808 which applies a codebook gain g_(p) to the informationfrom the adaptive codebook 800. The output of the amplifier 808 isconnected to an input of a combiner 810. Furthermore, a random noisegenerator 804 together with a fixed codebook 802 provides codebookinformation to a further amplifier g_(c). The amplifier g_(c) indicatedat 806 applies the gain factor g_(c), which is the fixed codebook gain,to the information provided by the fixed codebook 802 together with therandom noise generator 804. The output of the amplifier 806 is thenadditionally input into the combiner 810. The combiner 810 adds theresult of both codebooks amplified by the corresponding codebook gainsto obtain a combination signal which is then input into an LPC synthesisblock 814. The LPC synthesis block 814 is controlled by replacementrepresentation which is generated as discussed before.

This conventional procedure has certain drawbacks.

In order to cope with changing signal characteristics or in order toconverge the LPC envelope towards background noise like-properties, theLPC is changed during concealment by extra/interpolation with some otherLPC vectors. There is no possibility to precisely control the energyduring concealment. While there is the chance to control the codebookgains of the various codebooks, the LPC will implicitly influence theoverall level or energy (even frequency dependent).

It might be envisioned to fade out to a distinct energy level (e.g.background noise level) during burst frame loss. This is not possiblewith state-of-the-art technology, even by controlling the codebookgains.

It is not possible to fade the noisy parts of the signal to backgroundnoise, while maintaining the possibility to synthesize tonal parts withthe same spectral property as before the frame loss.

SUMMARY

According to an embodiment, an apparatus for generating an errorconcealment signal may have: an LPC (linear prediction coding)representation generator for generating a replacement LPCrepresentation; an LPC synthesizer for filtering a codebook informationusing the replacement LPC representation to obtain a replacement signal,from which the error concealment signal is derived; and a noiseestimator for estimating a noise estimate during a reception of goodaudio frames, wherein the noise estimate depends on the good audioframes, and wherein the noise estimate is derived from a past decodedsignal, and wherein the LPC representation generator is configured touse the noise estimate estimated by the noise estimator in generatingthe replacement LPC representation.

According to another embodiment, a method for generating an errorconcealment signal may have the steps of: generating a replacement LPCrepresentation; filtering a codebook information using the replacementLPC representation to obtain a replacement signal, from which the errorconcealment signal is derived; and estimating a noise estimate during areception of good audio frames, wherein the noise estimate depends onthe good audio frames, and wherein the noise estimate is derived from apast decoded signal, and wherein the noise estimate estimated by theestimating is used in generating the replacement LPC representation.

Another embodiment may have a non-transitory digital storage mediumhaving a computer program stored thereon to perform the method forgenerating an error concealment signal, the method having the steps of:generating a replacement LPC representation; filtering a codebookinformation using the replacement LPC representation to obtain areplacement signal, from which the error concealment signal is derived;and estimating a noise estimate during a reception of good audio frames,wherein the noise estimate depends on the good audio frames, and whereinthe noise estimate is derived from a past decoded signal, and whereinthe noise estimate estimated by the estimating is used in generating thereplacement LPC representation, when said computer program is run by acomputer.

In an aspect of the present invention, the apparatus for generating anerror concealment signal comprises an LPC representation generator forgenerating a first replacement LPC representation and a different,second replacement LPC representation. Furthermore, an LPC synthesizeris provided for filtering a first codebook information using the firstreplacement LPC representation to obtain a first replacement signal andfor filtering a second different codebook information using the secondreplacement LPC representation to obtain a second replacement signal.The outputs of the LPC synthesizer are combined by a replacement signalcombiner combining the first replacement signal and the secondreplacement signal to obtain the error concealment signal.

The first codebook is an adaptive codebook for providing the firstcodebook information and the second codebook as a fixed codebook forproviding the second codebook information. In other words, the firstcodebook represents the tonal part of the signal and the second or fixedcodebook represents the noisy part of the signal and therefore can beconsidered to be a noise codebook.

The first codebook information for the adaptive codebook is generatedusing a mean value of last good LPC representations, the last goodrepresentation and a fading value. Furthermore, the LPC representationfor the second or fixed codebook is generated using the last good LPCrepresentation fading value and a noise estimate. Depending on theimplementation, the noise estimate can be a fixed value, an offlinetrained value or it can be adaptively derived from a signal preceding anerror concealment situation.

An LPC gain calculation for calculating an influence of a replacementLPC representation is performed and this information is then used inorder to perform a compensation so that the power or loudness or,generally, an amplitude-related measure of the synthesis signal issimilar to the corresponding synthesis signal before the errorconcealment operation.

In a further aspect, an apparatus for generating an error concealmentsignal comprises an LPC representation generator for generating one ormore replacement LPC representations. Furthermore, the gain calculatoris provided for calculating the gain information from the LPCrepresentation and a compensator is then additionally provided forcompensating a gain influence of the replacement LPC representation andthis gain compensation operates using the gain operation provided by thegain calculator. An LPC synthesizer then filters a codebook informationusing the replacement LPC representation to obtain the error concealmentsignal, wherein the compensator is configured for weighting the codebookinformation before being synthesized by the LPC synthesizer or forweighting the LPC synthesis output signal. Thus, any gain or power oramplitude-related perceivable influence at the onset of an errorconcealment situation is reduced or eliminated.

This compensation is not only useful for individual LPC representationsas outlined in the above aspect, but is also useful in the case of usingonly a single LPC replacement representation together with a single LPCsynthesizer.

The gain values are determined by calculating impulse responses of thelast good LPC representation and a replacement LPC representation and byparticularly calculating an rms value over the impulse response of thecorresponding LPC representation over a certain time which is between 3and 8 ms and is advantageously 5 ms.

In an implementation, the actual gain value is determined by dividing anew rms value, i.e. an rms value for a replacement LPC representation byan rms value of good LPC representation.

The single or several replacement LPC representations is/are calculatedusing a background noise estimate which is a background noise estimatederived from the currently decoded signals in contrast to an offlinetrained vector simply predetermined noise estimate.

In a further aspect, an apparatus for generating a signal comprises anLPC representation generator for generating one or more replacement LPCrepresentations, and an LPC synthesizer for filtering a codebookinformation using the replacement LPC representation. Additionally, anoise estimator for estimating a noise estimate during a reception ofgood audio frames is provided, and this noise estimate depends on thegood audio frames. The representation generator is configured to use thenoise estimate estimated by the noise estimator in generating thereplacement LPC representation.

Spectral representation of a past decoded signal is process to provide anoise spectral representation or target representation. The noisespectral representation is converted into a noise LPC representation andthe noise LPC representation is the same kind of LPC representation asthe replacement LPC representation. ISF vectors or LSF vectors areadvantageous for the specific LPC-related processing procedures.

Estimate is derived using a minimum statistics approach with optimalsmoothing to a past decoded signal. This spectral noise estimate is thenconverted into a time domain representation. Then, a Levinson-Durbinrecursion is performed using a first number of samples of the timedomain representation, where the number of samples is equal to an LPCorder. Then, the LPC coefficients are derived from the result of theLevinson-Durbin recursion and this result is finally transformed in avector. The aspect of using individual LPC representations forindividual codebooks, the aspect of using one or more LPCrepresentations with a gain compensation and the aspect of using a noiseestimate in generating one or more LPC representations, which estimateis not an offline-trained vector but is a noise estimate derived fromthe past decoded signal are individually useable for obtaining animprovement with respect to conventional technology.

Additionally, these individual aspects can also be combined with eachother so that, for example, the first aspect and the second aspect canbe combined or the first aspect or the third aspect can be combined orthe second aspect and the third aspect can be combined to each other toprovide an even improved performance with respect to conventionaltechnology. Even more advantageously, all three aspects can be combinedwith each other to obtain improvements over conventional technology.Thus, even though the aspects are described by separate figures allaspects can be applied in combination with each other, as can be seen byreferring to the enclosed figures and description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1a illustrates an embodiment of the first aspect;

FIG. 1b illustrates a usage of an adaptive codebook;

FIG. 1c illustrates a usage of a fixed codebook in the case of a normalmode or a concealment mode;

FIG. 1d illustrates a flowchart for calculating the first LPCreplacement representation;

FIG. 1e illustrates a flowchart for calculating the second LPCreplacement representation;

FIG. 2 illustrates an overview over a decoder with error concealmentcontroller and noise estimator;

FIG. 3 illustrates a detailed representation of the synthesis filters;

FIG. 4 illustrates an embodiment combining the first aspect and thesecond aspect;

FIG. 5 illustrates a further embodiment combining the first and secondaspects;

FIG. 6 illustrates the embodiment combining the first and secondaspects;

FIG. 7a illustrates an embodiment for performing a gain compensation.

FIG. 7b illustrates a flowchart for performing a gain compensation;

FIG. 8 illustrates a conventional error concealment signal generator;

FIG. 9 illustrates an embodiment in accordance with the second aspectwith gain compensation;

FIG. 10 illustrates a further implementation of the embodiment of FIG.9;

FIG. 11 illustrates an embodiment of the third aspect using the noiseestimator;

FIG. 12a illustrates an implementation for calculating the noiseestimate;

FIG. 12b illustrates a further implementation for calculating the noiseestimate; and

FIG. 13 illustrates the calculation of a single LPC replacementrepresentation or individual LPC replacement representations forindividual codebooks using a noise estimate and applying a fadingoperation.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention relate to controlling the level ofthe output signal by means of the codebook gains independently of anygain change caused by an extrapolated LPC and to control the LPC modeledspectral shape separately for each codebook. For this purpose, separateLPCs are applied for each codebook and compensation means are applied tocompensate for any change of the LPC gain during concealment.

Embodiments of the present invention as defined in the different aspectsor in combined aspects have the advantage of providing a high subjectivequality of speech/audio in case of one or more data packets not beingcorrectly or not being received at all at the decoder side.

Furthermore, the embodiments compensate the gain differences betweensubsequent LPCs during concealment, which might result from the LPCcoefficients being changed over time, and therefore unwanted levelchanges are avoided.

Furthermore, embodiments are advantageous in that during concealment twoor more sets of LPC coefficients are used to independently influence thespectral behavior of voiced and unvoiced speech parts and also tonal andnoise-like audio parts.

All aspects of the present invention provide an improved subjectiveaudio quality.

According to one aspect of this invention, the energy is preciselycontrolled during the interpolation. Any gain that is introduced bychanging the LPC is compensated.

According to another aspect of this invention, individual LPCcoefficient sets are utilized for each of the codebook vectors. Eachcodebook vector is filtered by its corresponding LPC and the individualfiltered signals are just afterwards summed up to obtain the synthesizedoutput.

In contrast, state-of-the-art technology first adds up all excitationvectors (being generated from different codebooks) and just then feedsthe sum to a single LPC filter.

According to another aspect, a noise estimate is not used, for exampleas an offline-trained vector, but is actually derived from the pastdecoded frames so that, after a certain amount of erroneous or missingpackets/frames, a fade-out to the actual background noise rather thanany predetermined noise spectrum is obtained. This particularly resultsin a feeling of acceptance at a user side, but to the fact that evenwhen an error situation occurs, the signal provided by the decoder aftera certain number of frames is related to the preceding signal. However,the signal provided by a decoder in the case of a certain number of lostor erroneous frames is a signal completely unrelated to the signalprovided by the decoder before an error situation.

Applying gain compensation for the time-varying gain of the LPC allowsthe following advantages:

It compensates any gain that is introduced by changing the LPC.

Hence, the level of the output signal can be controlled by the codebookgains of the various codebooks. This allows for a pre-determinedfade-out by eliminating any unwanted influence by the interpolated LPC.

Using a separate set of LPC coefficients for each codebook used duringconcealment allows the following advantages:

It creates the possibility to influence the spectral shape of tonal andnoise like parts of the signal separately.

It gives the chance to play out the voiced signal part almost unchanged(e.g. desired for vowels), while the noise part may quickly beconverging to background noise.

It gives the chance to conceal voiced parts, and fade out the voicedpart with arbitrary fading speed (e.g. fade out speed dependent fromsignal characteristics), while simultaneously maintaining the backgroundnoise during concealment. State-of-the-art codecs usually suffer from avery clean voiced concealment sound.

It provides means to fade to background noise during concealmentsmoothly, by fading out the tonal parts without changing the spectralproperties, and fading the noise like parts to the background spectralenvelope.

FIG. 1a illustrates an apparatus for generating an error concealmentsignal 111. The apparatus comprises an LPC representation generator 100for generating a first replacement representation and additionally forgenerating a second replacement LPC representation. As outlined in FIG.1a , the first replacement representation is input into an LPCsynthesizer 106 for filtering a first codebook information output by afirst codebook 102 such as an adaptive codebook 102 to obtain a firstreplacement signal at the output of block 106. Furthermore, the secondreplacement representation generated by the LPC representation generator100 is input into the LPC synthesizer for filtering a second differentcodebook information provided by a second codebook 104 which is, forexample, a fixed codebook, to obtain a second replacement signal at theoutput of block 108. Both replacement signals are then input into areplacement signal combiner 110 for combining the first replacementsignal and the second replacement signal to obtain the error concealmentsignal 111. Both LPC synthesizers 106, 108 can be implemented in asingle LPC synthesizer block or can be implemented as separate LPCsynthesizer filters. In other implementations, both LPC synthesizerprocedures can be implemented by two LPC filters actually beingimplemented and operating in parallel. However, the LPC synthesis canalso be an LPC synthesis filter and a certain control so that the LPCsynthesis filter provides an output signal for the first codebookinformation and the first replacement representation and then,subsequent to this first operation, the control provides the secondcodebook information and the second replacement representation to thesynthesis filter to obtain the second replacement signal in a serialway. Other implementations for the LPC synthesizer apart from a singleor several synthesis blocks are clear for those skilled in the art.

Typically, the LPC synthesis output signals are time domain signals andthe replacement signal combiner 110 performs a synthesis output signalcombination by performing a synchronized sample-by-sample addition.However, other combinations, such as a weighted sample-by-sampleaddition or a frequency domain addition or any other signal combinationcan be performed by the replacement signal combiner 110 as well.

Furthermore, the first codebook 102 is indicated as comprising anadaptive codebook and the second codebook 104 is indicated as comprisinga fixed codebook. However, the first codebook and the second codebookcan be any codebooks such as a predictive codebook as the first codebookand a noise codebook as the second codebook. However, other codebookscan be glottal pulse codebooks, innovative codebooks, transitioncodebooks, hybrid codebooks consisting of predictive and transformparts, codebooks for individual voice generators such asmales/females/children or codebooks for different sounds such as foranimal sounds, etc.

FIG. 1b illustrates a representation of an adaptive codebook. Theadaptive codebook is provided with a feedback loop 120 and receives, asan input, a pitch lag 118. The pitch lag can be a decoded pitch lag inthe case of a good received frame/packet. However, if an error situationis detected indicating an erroneous or missing frame/packet, then anerror concealment pitch lag 118 is provided by the decoder and inputinto the adaptive codebook. The adaptive codebook 102 can be implementedas a memory storing the fed back output values provided via the feedbackline 120 and, depending on the applied pitch lag 118, a certain amountof sampling values is output by the adaptive codebook.

Furthermore, FIG. 1c illustrates a fixed codebook 104. In the case ofthe normal mode, the fixed codebook 104 receives a codebook index and,in response to the codebook index, a certain codebook entry 114 isprovided by the fixed codebook as codebook information. However, if aconcealment mode is determined, a codebook index is not available. Then,a noise generator 112 provided within the fixed codebook 104 isactivated which provides a noise signal as the codebook information 116.Depending on the implementation, the noise generator may provide arandom codebook index. However, it is advantageous that a noisegenerator actually provides a noise signal rather than a random codebookindex. The noise generator 112 may be implemented as a certain hardwareor software noise generator or can be implemented as noise tables or acertain “additional” entry in the fixed codebook which has a noiseshape. Furthermore, combinations of the above procedures are possible,i.e. a noise codebook entry together with a certain post-processing.

FIG. 1d illustrates a procedure for calculating a first replacement LPCrepresentation in the case of an error. Step 130 illustrates thecalculation of a mean value of LPC representations of two or more lastgood frames. Three last good frames are advantageous. Thus, a mean valueover the three last good frames is calculated in block 130 and providedto block 136. Furthermore, a stored last good frame LPC information isprovided in step 132 and additionally provided to the block 136.Furthermore, a fading factor 134 is determined in block 134. Then,depending on the last good LPC information, depending on the mean valueof the LPC information of the last good frame and depending on thefading factor of block 134, the first replacement representation 138 iscalculated.

For the state-of-the-art just one LPC is applied. For the newly proposedmethod, each excitation vector, which is generated by either theadaptive or the fixed codebook, is filtered by its own set of LPCcoefficients. The derivation of the individual ISF vectors is asfollows:

Coefficient set A (for filtering the adaptive codebook) is determined bythis formula:

$\begin{matrix}{{isf}^{\mspace{11mu} \prime} = \frac{{isf}^{\mspace{11mu} {- 2}} + {isf}^{\mspace{11mu} {- 3}} + {isf}^{\mspace{11mu} {- 4}}}{3}} & \left( {{block}\mspace{14mu} 136} \right) \\{{isf}_{A}^{\; {- 1}} = {{{alpha}_{A} \cdot {isf}^{\mspace{11mu} {- 2}}} + {\left( {1 - {alpha}} \right) \cdot {isf}^{\mspace{11mu} \prime}}}} & \left( {{block}\mspace{14mu} 136} \right)\end{matrix}$

where alpha_(A) is a time varying adaptive fading factor which maydepend on signal stability, signal class, etc. is f^(−x) are the ISFcoefficients, where x denotes the frame number, relative to the end ofthe current frame: x=−1 denotes the first lost ISF, x=−2 the last good,x=−3 second last good and so on. This leads to fading the LPC which isused for filtering the tonal part, starting from the last correctlyreceived frame towards the average LPC (averaged over three of the lastgood 20 ms frames). The more frames get lost, the closer the ISF, whichis used during concealment, will be to this short term average ISFvector (isf′). Generally, it is to be noted that ISF stands for valuesin an ISF domain or in an LSF domain. Hence, the same calculations orslightly different calculations can also be performed in the LSF domainrather than in the ISF domain or any other similar domain.

FIG. 1e illustrates a procedure for calculating the second replacementrepresentation. In block 140, a noise estimate is determined. Then, inblock 142, a fading factor is determined. Additionally, in block 144,the last good frame is LPC information which has been stored before isprovided. Then, in block 146, a second replacement representation iscalculated. Advantageously, a coefficient set B (for filtering the fixedcodebook) is determined by this formula:

isf _(B) ⁻¹=alpha_(B) ·isf ⁻²+(1−beta)·isf ^(cng) (block 146)

where isf^(cng) is the ISF coefficient set derived from a backgroundnoise estimate and alpha_(B) is the time-varying fading speed factorwhich is signal dependent. The target spectral shape is derived bytracing the past decoded signal in the FFT domain (power spectrum),using a minimum statistics approach with optimal smoothing, similar to[3]. This FFT estimate is converted to the LPC representation bycalculating the auto-correlation by doing inverse FFT and then usingLevinson-Durbin recursion to calculate LPC coefficients using the firstN samples of the inverse FFT, where N is the LPC order. Hence, theLevinson Durbin recursion is calculated on auto-correlated values or thetime domain representation based on which the recursion is calculatedcomprises an inverse of a squared Fourier Transform (e.g. FFT) spectrum.

This LPC is then converted into the ISF domain to retrieve isf^(cng).Alternatively—if such tracing of the background spectral shape is notavailable—the target spectral shape might also be derived based on anycombination of an offline trained vector and the short-term spectralmean, as it is done in G.718 for the common target spectral shape.

The fading factors A and α_(B) are determined depending on the decodedaudio signal, i.e., depending on the decoded audio signal before theoccurrence of an error. The fading factor may depend on signalstability, signal class, etc. Thus, is the signal is determined to be aquite noisy signal, then the fading factor is determined in such a waythat the fading factor decreases, from time to time, more quickly thancompared to a situation where a signal is quite tonal. In thissituation, the fading factor decreases from one time frame to next timeframe by a reduced amount. This makes sure that the fading out from thelast good frame to the mean value of the last three good frames takesplace more quickly in the case of noisy signals compared to non-noisy ortonal signals, where the fading out speed is reduced. Similar procedurescan be performed for signal classes. For voiced signals, a fading outcan be performed slower than for unvoiced signals or for music signals acertain fading speed can be reduced compared to further signalcharacteristics and corresponding determinations of the fading factorcan be applied.

As discussed in the context of FIG. 1e , a different fading factor α_(B)can be calculated for the second codebook information. Thus, thedifferent codebook entries can be provided with a different fadingspeed. Thus, a fading out to the noise estimate as f^(cng) can be setdifferently from the fading speed from the last good frame ISFrepresentation to the mean ISF representation as outlined in block 136of FIG. 1 d.

FIG. 2 illustrates an overview of an implementation. An input linereceives, for example, from a wireless input interface or a cableinterface packets or frames of an audio signal. The data on the inputline 202 is provided to a decoder 204 and at the same time to an errorconcealment controller 200. The error concealment controller determineswhether received packet or frames are erroneous or missing. If this isdetermined, the error concealment controller inputs a control message tothe decoder 204. In the FIG. 2 implementation, a “1” message on thecontrol line CTRL signals that the decoder 204 is to operate in theconcealment mode. However, if the error concealment controller does notfind an error situation, then the control line CTRL carries a “0”message indicating a normal decoding mode as indicated in table 210 ofFIG. 2. The decoder 204 is additionally connected to a noise estimator206. During the normal decoding mode, the noise estimator 206 receivesthe decoded audio signal via a feedback line 208 and determines a noiseestimate from the decoded signal. However, when the error concealmentcontroller indicates a change from the normal decoding mode to theconcealment mode, the noise estimator 206 provides the noise estimate tothe decoder 204 so that the decoder 204 can perform an error concealmentas discussed in the preceding and the next figures. Thus, the noiseestimator 206 is additionally controlled by the control line CTRL fromthe error concealment controller to switch, from the normal noiseestimation mode in the normal decoding mode to the noise estimateprovision operation in the concealment mode.

FIG. 4 illustrates an embodiment of the present invention in the contextof a decoder, such as the decoder 204 of FIG. 2, having an adaptivecodebook 102 and additionally having a fixed codebook 104. In the normaldecoding mode indicated by a control line data “0” as discussed in thecontext of the table 210 in FIG. 2, the decoder operates as illustratedin FIG. 8, when item 804 is neglected. Thus, the correctly receivedpacket comprises a fixed codebook index for controlling the fixedcodebook 802, a fixed codebook gain g_(c) for controlling amplifier 806and an adaptive codebook g_(p) in order to control the amplifier 808.Furthermore, the adaptive codebook 800 is controlled by the transmittedpitch lag and the switch 812 is connected so that the adaptive codebookoutput is fed back into the input of the adaptive codebook. Furthermore,the coefficients for the LPC synthesis filter 804 are derived from thetransmitted data.

However, if an error concealment situation is detected by the errorconcealment controller 202 of FIG. 2, the error concealment procedure isinitiated in which, in contrast to the normal procedure, two synthesisfilters 106, 108 are provided. Furthermore, the pitch lag for theadaptive codebook 102 is generated by an error concealment device.Additionally, the adaptive codebook gain g_(p) and the fixed codebookgain g_(c) are also synthesized by an error concealment procedure asknown in the art in order to correctly control the amplifiers 402, 404.

Furthermore, depending on the signal class, a controller 409 controlsthe switch 405 in order to either feedback a combination of bothcodebook outputs (subsequent to the application of the correspondingcodebook gain) or to only feedback the adaptive codebook output.

In accordance with an embodiment, the data for the LPC synthesis filterA 106 and the data for the LPC synthesis filter B 108 is generated bythe LPC representation generator 100 of FIG. 1a and additionally a gaincorrection is performed by the amplifiers 406, 408. To this end, thegain compensation factors g_(A) and g_(B) are calculated in order tocorrectly drive the amplifiers 408, 406 so that any gain influencegenerated by the LPC representation is stopped. Finally, the output ofthe LPC synthesis filters A, B indicated by 106 and 108 are combined bythe combiner 110, so that the error concealment signal is obtained.

Subsequently, the switching from the normal mode to the concealment modeon one hand and from the concealment mode back to the normal mode isdiscussed.

The transition from one common to several separate LPCs when switchingfrom clean channel decoding to concealment does not cause anydiscontinuities, as the memory state of the last good LPC may be used toinitialize each AR or MA memory of the separate LPCs. When doing so, asmooth transition from the last good to the first lost frame is ensured.

When switching from concealment to clean channel decoding (recoveryphase), the approach of the separate LPCs introduces the challenge tocorrectly update the internal memory state of the single LPC filterduring clean-channel decoding (usually AR (auto-regressive) models areused). Just using the AR memory of one LPC or an averaged AR memorywould lead to discontinuities at the frame border between the last lostand the first good frame. In the following a method is described toovercome deal with this challenge:

A small portion of all excitation vectors (suggestion: 5 ms) is added atthe end of any concealed frame. This summed excitation vector may thenbe fed to the LPC which would be used for recovery. This is shown inFIG. 5. Depending on the implementation it is also possible to sum upthe excitation vectors after the LPC gain compensation.

It is advisable to start at frame end minus 5 ms, setting the LPC ARmemory to zero, derive the LPC synthesis by using any of the individualLPC coefficient sets and save the memory state at the very end of theconcealed frame. If the next frame is correctly received, this memorystate may then be used for recovery (meaning: used for initializing thestart-of-frame LPC memory), otherwise it is discarded. This memory hasto be additionally introduced; it has to be handled separately from anyof the used LPC AR memories of the concealment used during concealment.

Another solution for recovery is to use the method LPC0, known from USAC[4].

Subsequently, FIG. 5 is discussed in more detail. Generally, theadaptive codebook 102 can be termed to be a predictive codebook asindicated in FIG. 5 or can be replaced by a predictive codebook.Furthermore, the fixed codebook 104 can be replaced or implemented asthe noise codebook 104. The codebook gains g_(p) and g_(c), in order tocorrectly drive the amplifiers 402, 404 are transmitted, in the normalmode, in the input data or can be synthesized by an error concealmentprocedure in the error concealment case. Furthermore, a third codebook412, which can be any other codebook, is used which additionally has anassociated codebook gain g_(r) as indicated by amplifier 414. In anembodiment, an additional LPC synthesis by a separate filter controlledby an LPC replacement representation for the other codebook isimplemented in block 416. Furthermore, a gain correction g_(c) isperformed in a similar way as discussed in the context of g_(A) andg_(B), as outlined.

Furthermore, the additional recovery LPC synthesizer X indicated at 418is shown which receives, as an input, a sum of at least a small portionof all excitation vectors such as 5 ms. This excitation vector is inputinto the LPC synthesizer X 418 memory states of the LPC synthesis filterX.

Then, when a switchback from the concealment mode to the normal modeoccurs, the single LPC synthesis filter is controlled by copying theinternal memory states of the LPC synthesis filter X into this singlenormal operating filter and additionally the coefficients of the filterare set by the correctly transmitted LPC representation.

FIG. 3 illustrates a further, more detailed implementation of the LPCsynthesizer having two LPC synthesis filters 106, 108. Each filter is,for example, an FIR filter or an IIR filter having filter taps 304, 306and filter-internal memories 304, 308. The filter taps 302, 306 arecontrolled by the corresponding LPC representation correctly transmittedor the corresponding replacement LPC representation generated by the LPCrepresentation generator such as 100 of FIG. 1a . Furthermore, a memoryinitializer 320 is provided. The memory initializer 320 receives thelast good LPC representation and, when switch over to the errorconcealment mode is performed, the memory initializer 320 provides thememory states of the single LPC synthesis filter to the filter-internalmemories 304, 308. In particular, the memory initializer receives,instead of the last good LPC representation or in addition to the lastgood LPC representation, the last good memory states, i.e. the internalmemory states of the single LPC filter in the processing, andparticularly after the processing of the last good frame/packet.

Additionally, as already discussed in the context of FIG. 5, the memoryinitializer 320 can also be configured to perform the memoryinitialization procedure for a recovery from an error concealmentsituation to the normal non-erroneous operating mode. To this end, thememory initializer 320 or a separate future LPC memory initializer isconfigured for initializing a single LPC filter in the case of arecovery from an erroneous or lost frame to a good frame. The LPC memoryinitializer is configured for feeding at least a portion of a combinedfirst codebook information and second codebook information or at least aportion of a combined weighted first codebook information or a weightedsecond codebook information into a separate LPC filter such as LPCfilter 418 of FIG. 5. Additionally, the LPC memory initializer isconfigured for saving memory states obtained by processing the fed invalues. Then, when a subsequent frame or packet is a good frame orpacket, the single LPC filter 814 of FIG. 8 for the normal mode isinitialized using the saved memory states, i.e. the states from filter418. Furthermore, as outlined in FIG. 5, the filter coefficients for thefilter can be either the coefficient for LPC synthesis filter 106 or LPCsynthesis filter 108 or LPC synthesis filter 416 or a weighted orunweighted combination of those coefficients.

FIG. 6 illustrates a further implementation with gain compensation. Tothis end, the apparatus for generating an error concealment signalcomprises a gain calculator 600 and a compensator 406, 408, which hasalready been discussed in the context of FIG. 4 (406, 408) and FIG. 5(406, 408, 409). In particular, the LPC representation calculator 100outputs the first replacement LPC representation and the secondreplacement LPC representation to a gain calculator 600. The gaincalculator then calculates a first gain information for the firstreplacement LPC representation and the second gain information for thesecond LPC replacement representation and provides this data to thecompensator 406, 408, which receives, in addition to the first andsecond codebook information, as outlined in FIG. 4 or FIG. 5, the LPC ofthe last good frame/packet/block. Then, the compensator outputs thecompensated signal. The input into the compensator can either be anoutput of amplifiers 402, 404, an output of the codebooks 102, 104 or anoutput of the synthesis blocks 106, 108 in the embodiment of FIG. 4.

Compensator 406, 408 partly or fully compensates a gain influence of thefirst replacement LPC in the first gain information and compensates again influence of the second replacement LPC representation using thesecond gain information.

In an embodiment, the calculator 600 is configured to calculate a lastgood power information related to a last good LPC representation beforea start of the error concealment. Furthermore, the gain calculator 600calculates a first power information for the first replacement LPCrepresentation, a second power information for the second LPCrepresentation, the first gain value using the last good powerinformation and the first power information, and a second gain valueusing the last good power information and the second power information.Then, the compensation is performed in the compensator 406, 408 usingthe first gain value and using the second gain value. Depending on theinformation, however, the calculation of the last good power informationcan also be performed, as illustrated in the FIG. 6 embodiment, by thecompensator directly. However, due to the fact that the calculation ofthe last good power information is basically performed in the same wayas the first gain value for the first replacement representation and thesecond gain value for the second replacement LPC representation, it isadvantageous to perform the calculation of all gain values in the gaincalculator 600 as illustrated by the input 601.

In particular, the gain calculator 600 is configured to calculate fromthe last good LPC representation or the first and second LPC replacementrepresentations an impulse response and to then calculate an rms (rootmean square) value from the impulse response to obtain the correspondentpower information in the gain compensation, each excitation vectoris—after being gained by the corresponding codebook gain—again amplifiedby the gains: g_(A) or g_(B). These gains are determined by calculatingthe impulse response of the currently used LPC and then calculating therms:

${rms}_{new} = \sqrt{\sum\limits_{t = {0{ms}}}^{5\; {ms}}{{imp\_ resp}^{2}(t)}}$

The result is then compared to the rms of the last correctly receivedLPC and the quotient is used as gain factor in order to compensate forenergy increase/loss of LPC interpolation:

$g = \frac{{rms}_{old}}{{rms}_{new}}$

This procedure can be seen as a kind of normalization. It compensatesthe gain, which is caused by LPC interpolation.

Subsequently, FIGS. 7a and 7b are discussed in more detail to illustratethe apparatus for generating an error concealment signal or the gaincalculator 600 or the compensator 406, 408 calculates the last goodpower information as indicated at 700 in FIG. 7a . Furthermore, the gaincalculator 600 calculates the first and second power information for thefirst and second LPC replacement representation as indicated at 702.Then, as illustrated by 704, the first and the second gain values arecalculated by the gain calculator 600. Then, the codebook information orthe weighted codebook information or the LPC synthesis output iscompensated using these gain values as illustrated at 706. Thiscompensation is done by the amplifiers 406, 408.

To this end, several steps are performed in an embodiment as illustratedin FIG. 7b . In step 710, an LPC representation, such as the first orsecond replacement LPC representation or the last good LPCrepresentation is provided. In step 712 the codebook gains are appliedto the codebook information/output as indicated by block 402, 404.Furthermore, in step 716, impulse responses are calculated from thecorresponding LPC representations. Then, in step 718, an rms value iscalculated for each impulse response and in block 720 the correspondinggain is calculated using an old rms value and a new rms value and thiscalculation is done by dividing the old rms value by the new rms value.Finally, the result of block 720 is used to compensate the result ofstep 712 in order to finally obtained the compensated results asindicated at step 714.

Subsequently, a further aspect is discussed, i.e. an implementation foran apparatus for generating an error concealment signal which ha the LPCrepresentation generator 100 generating only a single replacement LPCrepresentation, such as for the situation illustrated in FIG. 8. Incontrast to FIG. 8, however, the embodiment illustrating a furtheraspect in FIG. 9 comprises the gain calculator 600 and the compensator406, 408. Thus, any gain influence by the replacement LPC representationgenerated by the LPC representation generator is compensated for. Inparticular, this gain compensation can be performed on the input side ofthe LPC synthesizer as illustrated in FIG. 9 by compensator 406, 408 nor can be alternatively performed to the output of the LPC synthesizeras illustrated by the compensator 900 in order to finally obtain theerror concealment signal. Thus, the compensator 406, 408, 900 isconfigured for weighting the codebook information or an LPC synthesisoutput signal provided by the LPC synthesizer 106, 108.

The other procedures for the LPC representation generator, the gaincalculator, the compensator and the LPC synthesizer can be performed inthe same way as discussed in the context of FIGS. 1a to 8.

As has been outlined in the context of FIG. 4, the amplifier 402 and theamplifier 406 perform two weighting operations in series to each other,particularly in the case where not the sum of the multiplier output 402,404 is fed back into the adaptive codebook, but where only the adaptivecodebook output is fed back, i.e. when the switch 405 is in theillustrated position or the amplifier 404 and the amplifier 408 performtwo weighting operations in series. In an embodiment, illustrated inFIG. 10, these two weighting operations can be performed in a singleoperation. To this end, the gain calculator 600 provides its outputg_(p) or g_(c) to a single value calculator 1002. Furthermore, acodebook gain generator 1000 is implemented in order to generate aconcealment codebook gain as known in the art. The single valuecalculator 1002 then calculators a product between g_(p) and g_(A) inorder to obtain the single value. Furthermore, for the second branch,the single value calculator 1002 calculates a product between g_(A) org_(B) in order to provide the single value for the lower branch in FIG.4. A further procedure can be performed for the third branch havingamplifiers 414, 409 of FIG. 5.

Then a manipulator 1004 is provided which together performs theoperations of for example amplifiers 402, 406 to the codebookinformation of a single codebook or to the codebook information of twoor more codebooks in order to finally obtain a manipulated signal suchas a codebook signal or a concealment signal, depending on whether themanipulator 1004 is located before the LPC synthesizer in FIG. 9 orsubsequent to the LPC synthesizer of FIG. 9. FIG. 11 illustrates a thirdaspect, in which the LPC representation generator 100, the LPCsynthesizer 106, 108 and the additional noise estimator 206, which hasalready been discussed in the context of FIG. 2, are provided. The LPCsynthesizer 106, 108 receives codebook information and a replacement LPCrepresentation. The LPC representation is generated by the LPCrepresentation generator using the noise estimate from the noiseestimator 206, and the noise estimator 206 operates by determining thenoise estimate from the last good frames. Thus, the noise estimatedepends on the last good audio frames and the noise estimate isestimated during a reception of good audio frames, i.e. in the normaldecoding mode indicated by “0” on the control line of FIG. 2 and thisnoise estimate generated during the normal decoding mode is then appliedin the concealment mode as illustrated by the connection of blocks 206and 204 in FIG. 2.

The noise estimator is configured to process a spectral representationof a past decoded signal to provide a noise spectral representation andto convert the noise spectral representation into a noise LPCrepresentation, where the noise LPC representation is the same kind ofan LPC representation as the replacement LPC representation. Thus, whenthe replacement LPC representation is in the ISF-domain representationor an ISF vector, then the noise LPC representation additionally is anISF vector or ISF representation.

Furthermore, the noise estimator 206 is configured to apply a minimumstatistics approach with optimal smoothing to a past decoded signal toderive the noise estimate. For this procedure, it is advantageous toperform the procedure illustrated in [3]. However, other noiseestimation procedures relying on, for example, suppression of tonalparts compared to non-tonal parts in a spectrum in order to filter outthe background noise or noise in an audio signal can be applied as wellfor obtaining the target spectral shape or noise spectral estimate.

Thus, in one embodiment, a spectral noise estimate is derived from apast decoded signal and the spectral noise estimate is then convertedinto an LPC representation and then into an ISF domain to obtain thefinal noise estimate or target spectral shape.

FIG. 12a illustrates an embodiment. In step 1200, the past decodedsignal is obtained, as for example illustrated in FIG. 2 by the feedbackloop 208. In step 1202, a spectral representation, such as a FastFourier transform (FFT) representation is calculated. Then, in step 1204a target spectral shape is derived such as by the minimum statisticsapproach with optimal smoothing or by any other noise estimatorprocessing. Then, the target spectral shape is converted into an LPCrepresentation as indicated by block 1206 and finally the LPCrepresentation is converted to an ISF factor as outlined by block 1208in order to finally obtain the target spectral shape in the ISF domainwhich can then be directly used by the LPC representation generator forgenerating a replacement LPC representation. In the equations of thisapplication, the target spectral shape in the ISF domain is indicated as“ISF^(cng)”.

In an embodiment illustrated in FIG. 12b , the target spectral shape isderived for example by a minimum statistics approach and optimalsmoothing. Then, in step 1212, a time domain representation iscalculated by applying an inverse FFT, for example, to the targetspectral shape. Then, LPC coefficients are calculated by usingLevinson-Durbin recursion. However, the LPC coefficients calculation ofblock 1214 can also be performed by any other procedure apart from thementioned Levinson-Durbin recursion. Then, in step 1216, the final ISFfactor is calculated to obtain the noise estimate ISF^(cng) to be usedby the LPC representation generator 100.

Subsequently, FIG. 13 is discussed for illustrating the usage of thenoise estimate in the context of the calculation of a single LPCreplacement representation 1308 for the procedure, for example,illustrated in FIG. 8 or for calculating individual LPC representationsfor individual codebooks as indicated by block 1310 for the embodimentillustrated in FIG. 1.

In step 1300, a mean value of two or three last good frames iscalculated. In step 1302, the last good frame LPC representation isprovided. Furthermore, in step 1304, a fading factor is provided whichcan be controlled, for example, by a separate signal analyzer which canbe, for example, included in the error concealment controller 200 ofFIG. 2. Then, in step 1306, a noise estimate is calculated and theprocedure in step 1306 can be performed by any of the proceduresillustrated in FIGS. 12a , 12 b.

In the context of calculating a single LPC replacement representation,the outputs of blocks 1300, 1304, 1306 are provided to the calculator1308. Then, a single replacement LPC representation is calculated insuch a way that subsequent to a certain number of lost or missing orerroneous frames/packets, the fading over to the noise estimate LPCrepresentation is obtained.

However, individual LPC representations for an individual codebook, suchas for the adaptive codebook and the fixed codebook, are calculated asindicated at block 1310, then the procedure as discussed before forcalculating ISF_(A) ⁻¹ (LPC A) on the hand and the calculation ofISF_(B) ⁻¹ (LPC B) is performed.

Although the present invention has been described in the context ofblock diagrams where the blocks represent actual or logical hardwarecomponents, the present invention can also be implemented by acomputer-implemented method. In the latter case, the blocks representcorresponding method steps where these steps stand for thefunctionalities performed by corresponding logical or physical hardwareblocks.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by (or using)a hardware apparatus, like for example, a microprocessor, a programmablecomputer or an electronic circuit. In some embodiments, some one or moreof the most important method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disc, a DVD, a Blu-Ray, a CD, a ROM, a PROM, and EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate (or are capable of cooperating)with a programmable computer system such that the respective method isperformed. Therefore, the digital storage medium may be computerreadable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may, for example, be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive method is, therefore, a datacarrier (or a non-transitory storage medium such as a digital storagemedium, or a computer-readable medium) comprising, recorded thereon, thecomputer program for performing one of the methods described herein. Thedata carrier, the digital storage medium or the recorded medium aretypically tangible and/or non-transitory.

A further embodiment of the invention method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may, for example, be configured to be transferredvia a data communication connection, for example, via the internet.

A further embodiment comprises a processing means, for example, acomputer or a programmable logic device, configured to, or adapted to,perform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer (for example, electronically oroptically) a computer program for performing one of the methodsdescribed herein to a receiver. The receiver may, for example, be acomputer, a mobile device, a memory device or the like. The apparatus orsystem may, for example, comprise a file server for transferring thecomputer program to the receiver.

In some embodiments, a programmable logic device (for example, a fieldprogrammable gate array) may be used to perform some or all of thefunctionalities of the methods described herein. In some embodiments, afield programmable gate array may cooperate with a microprocessor inorder to perform one of the methods described herein. Generally, themethods are performed by any hardware apparatus.

While this invention has been described in terms of several advantageousembodiments, there are alterations, permutations, and equivalents whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andcompositions of the present invention. It is therefore intended that thefollowing appended claims be interpreted as including all suchalterations, permutations, and equivalents as fall within the truespirit and scope of the present invention.

REFERENCES

[1] ITU-T G.718 Recommendation, 2006

[2] Kazuhiro Kondo, Kiyoshi Nakagawa, “A Packet Loss Concealment MethodUsing Recursive Linear Prediction” Department of Electrical Engineering,Yamagata University, Japan.

[3] R. Martin, Noise Power Spectral Density Estimation Based on OptimalSmoothing and Minimum Statistics, IEEE Transactions on speech and audioprocessing, vol. 9, no. 5, July 2001

[4] Ralf Geiger et. al., Patent application US20110173011 A1, AudioEncoder and Decoder for Encoding and Decoding Frames of a Sampled AudioSignal

[5] 3GPP TS 26.190; Transcoding functions;—3GPP technical specification

1. Apparatus for generating an error concealment signal, comprising: anLPC (linear prediction coding) representation generator for generating areplacement LPC representation; an LPC synthesizer for filtering acodebook information using the replacement LPC representation to acquirea replacement signal, from which the error concealment signal isderived; and a noise estimator for estimating a noise estimate during areception of good audio frames, wherein the noise estimate depends onthe good audio frames, and wherein the noise estimate is derived from apast decoded signal, and wherein the LPC representation generator isconfigured to use the noise estimate estimated by the noise estimator ingenerating the replacement LPC representation.
 2. Apparatus of claim 1,wherein the noise estimator is configured to acquire the past decodedsignal, to calculate a spectral representation of the past decodedsignal, to derive a noise spectral representation from the spectralrepresentation of the past decoded signal, and to convert the noisespectral representation into a noise LPC representation, the noise LPCrepresentation being the same kind of LPC representation as thereplacement LPC representation.
 3. Apparatus of claim 1, wherein thereplacement LPC representation comprises a replacement factor, andwherein the noise estimator is configured to provide the noise estimateas a noise factor.
 4. Apparatus of claim 3, wherein the replacementfactor is an LSF (line spectral frequency) factor or an ISF (immitancespectral frequency) factor and wherein the noise factor is an LSF factoror an ISF factor.
 5. Apparatus of claim 1, wherein the noise estimatoris configured for applying a minimum statistics approach with optimalsmoothing to a past decoded signal to derive the noise estimate. 6.Apparatus of claim 1, wherein the noise estimator is configured toderive, from the past decoded signal, a spectral noise estimate, toconvert the spectral noise estimate into an LPC representation; and toconvert the LPC representation into an ISF domain or an LSF domain toacquire the noise estimate.
 7. Apparatus of claim 1, wherein the noiseestimator is configured to provide a spectral noise estimate; to convertthe spectral noise estimate into a time domain representation; and toperform a Levinson-Durbin recursion using the first N samples of thetime domain representation, wherein N corresponds to an LPC order of therepresentation.
 8. Apparatus of claim 7, wherein the time domainrepresentation comprises an inverse of a squared Fourier Transformspectrum.
 9. Apparatus of claim 1, wherein the LPC representationgenerator is configured to derive the replacement LPC representationusing the noise estimate and a last good LPC representation. 10.Apparatus of claim 1, wherein the LPC representation generator isconfigured to derive the replacement LPC representation using apreceding good LPC representation or a mean value of at least twopreceding good LPC representations, wherein the mean value or thepreceding good LPC representation is faded out such that, after a numberof erroneous or missing frames, the replacement LPC representationcorresponds to the noise estimate.
 11. Apparatus of claim 1, wherein theLPC representation generator is configured to generate a furtherreplacement LPC representation, wherein the apparatus further comprisesan adaptive codebook, wherein the LPC synthesizer is configured tofilter codebook information from a fixed codebook using the replacementLPC representation derived from the noise estimate to acquire a secondreplacement signal, and wherein the LPC synthesizer is configured tofilter a codebook information from the adaptive codebook using thefurther replacement LPC representation to acquire a first replacementsignal, wherein the LPC representation generator is configured tocalculate the further replacement LPC representation using a mean valueof at least two good LPC representations, and wherein the apparatusfurther comprises a replacement signal combiner configured to combiningthe first replacement signal and the second replacement signal toacquire the error concealment signal.
 12. Apparatus of claim 11, whereinthe LPC representation generator is configured to calculate thereplacement LPC representation based on the following equation:isf _(B) ⁻¹=α_(B) ·isf ⁻²+(1−α_(B))·isf ^(cng), wherein the LPCrepresentation generator is configured to calculate the furtherreplacement LPC representation based on the following equation:${isf}^{\mspace{11mu} \prime} = \frac{{isf}^{\mspace{11mu} {- 2}} + {isf}^{\mspace{11mu} {- 3}} + {isf}^{\mspace{11mu} {- 4}}}{3}$isf_(A)^( −1) = α_(A) ⋅ isf^(  −2) + (1 − α_(A)) ⋅ isf^(  ′), whereinα_(A) and α_(B) are time-varying fading factors, wherein isf⁻² is theLPC representation of the last good frame, wherein isf⁻³ is the LPCrepresentation of the second last good frame, wherein isf⁻⁴ is the LPCrepresentation of the third last good frame, wherein isf_(B) ⁻¹ is thereplacement LPC representation, and wherein isf_(A) ⁻¹ is the furtherreplacement LPC representation, wherein isf^(cng) is the noise estimate,and wherein ISF stands for values in an ISF domain or in an LSF domain.13. Apparatus of claim 1, further comprising a signal analyzer foranalyzing a signal characteristic of a signal received before anoccurrence of an error to be concealed, wherein the signal analyzer isconfigured to provide an analysis result, and wherein the LPCrepresentation generator is configured to use a time-varying fadingfactor, wherein the time-varying fading factor is determined dependingon the analysis result.
 14. Apparatus of claim 13, wherein the signalcharacteristic is a signal stability or a signal class, and wherein thetime-varying fading factor is determined so that the fading factordecreases to 0 in a shorter time for a signal being less stable or beingin a noise class compared to a signal being more stable or being in atonal class.
 15. Apparatus of claim 1, further comprising: a gaincalculator for calculating a gain information from the replacement LPCrepresentation; and a compensator for compensating a gain influence ofthe replacement LPC representation using the gain information, whereinthe compensator is configured for weighting a codebook information or anLPC synthesis output signal.
 16. A method for generating an errorconcealment signal, comprising: generating a replacement LPCrepresentation; filtering a codebook information using the replacementLPC representation to acquire a replacement signal, from which the errorconcealment signal is derived; and estimating a noise estimate during areception of good audio frames, wherein the noise estimate depends onthe good audio frames, and wherein the noise estimate is derived from apast decoded signal, and wherein the noise estimate estimated by theestimating is used in generating the replacement LPC representation. 17.A non-transitory digital storage medium having a computer program storedthereon to perform the method for generating an error concealmentsignal, the method comprising: generating a replacement LPCrepresentation; filtering a codebook information using the replacementLPC representation to acquire a replacement signal, from which the errorconcealment signal is derived; and estimating a noise estimate during areception of good audio frames, wherein the noise estimate depends onthe good audio frames, and wherein the noise estimate is derived from apast decoded signal, and wherein the noise estimate estimated by theestimating is used in generating the replacement LPC representation,when said computer program is run by a computer.